Shougo Mikadono is the only heir of an international business conglomerate, so of course his mother tells him that he's got to find his life partner before he leaves school. He's got to get married, basically. No, I don't buy that logic either. (Any bets on that marriage lasting eighteen months?)
Naturally, this is a fanservice harem anime and poor Shougo's going to find himself beset by girls who want to strip naked and jump him. The problem is that one's his sister. Same father, different mother. The matter was hushed up and not even Shougo's mother knows... but unfortunately this unknown sister wants to get inside Shougo's pants too.
GOOD NEWS: Shougo's horrified by the idea. (Yes, you can relax. This isn't one of those anime.) That's not much help, though, when you're pretty sure that one of your potential girlfriends is your sister, but not which one.
Believe it or not, this is a strong premise. It sidesteps all of the harem genre's traditional problems, which are...
QUESTION #1: "Why doesn't our hero just choose one girl and put a stop to the silliness? Or just have sex with all of them? Or..." ANSWER: well, d'oh.
QUESTION #2: "Why are harem protagonists so bland and insipid?" ANSWER: this one isn't. The show's effectively a detective story, with Shougo as a surprisingly sharp detective. There's lots of plot and revelations. Shougo's kept extremely busy investigating all the schemes, liars and (sometimes) outright malice around him. He's so good at analysis that it can be hard to keep up with his inferences, but the show plays fairer than you'd think. All the clues are given to us without misdirection, while the characters' feelings and reactions are presented so clearly that you'll start wondering if it's a double bluff. I guessed about half of the relevations before they came and the show didn't cheat.
(There are a couple of points he missed, though. Insufficient weight is given to the possibility of two hidden sisters, which is perfectly possible with a father like that. He also doesn't immediately label the DNA test sample he takes from Konoe in ep.7. That said, though, it's pleasing that the series acknowledges DNA tests in the first place. You might expect a harem show to put its fingers in its ears and shout "la la la" at something that could destroy its entire premise like that. Similarly, Shougo makes a habit of talking to the rest of the cast, asking straight questions and telling everyone what's going on. The only reason this isn't more effective is because not everyone tells the truth.)
QUESTION #3: "What do all those girls see in him?" ANSWER: goodness me, why might anyone target a kind, intelligent boy who's going to be richer than Croesus?
You'll see plenty of nudity and nipples, but the show's not driven by that. A fanservice anime would make that its selling point, but most of these episodes are clean.
The show's also funny.
Its only weak point, really, is the whole harem thing. This is forgivable in the TV series for all the aforementioned reasons, but in ep.12 Shougo cracks the sister mystery... and yet the show still ducks the question of which girl he'll choose. (I cared! The story's strong enough that I'd been following it properly, not just as dumb harem nonsense.)
There's still an OVA 13th episode, though... and that doesn't answer the question either. Instead, it leans heavily into incest fetishes, before backing away at the last minute and coughing shiftily. It has a date competition where the winner will become Shougo's girlfriend, as judged by his sister. The anti-resolution is frustrating. It's massively disappointing to see a good show descend to dumb silliness like this. It's pandering to harem genre fans, i.e. it'll annoy everyone else and clashes with the tone of the previous twelve episodes.
Mind you, the original light novel series (2010-13) hadn't finished when this anime was made (2012). Apparently the novels and anime have different sisters. That's okay, though, since the anime is internally consistent and punctilious about its clues. For example, the OVA explains a subtle dialogue point that only a super-detective (or hardcore nerd fan) would have noticed. Anyway, choosing Shougo's final romantic partner would have clashed with the still-ongoing novel series... but would that really have mattered, given the different sisters?
Also, more subjectively, I disagree with the series's romantic preferences. It's clear who we're meant to be cheering for, with an inner tier who got there first and have always got more attention (Konoe and Miyabi) and an outer tier of everyone else. Unfortunately, though, as far as I was concerned, Mizutani was the only possible choice. I'd throw things at anyone who disagreed. She's clever and super-efficient. You can have sensible conversations with her, in a way you couldn't with the others. She's been a life-saver. She's a bit weird, but that's okay. As for the others, though, they're perfectly nice and you wouldn't kick them out of bed, but... Miyabi is a tsundere, Kunitachi thinks she's God, Tendou is an airhead who doesn't care and anyone would run a mile from Sagara and her "GOD NO NO NO" Lyrical Sister Cafe.
As for Konoe, well, she's got big boobs and she's very feminine and agreeable... and that's it. There's nothing there. You could talk to her for months without finding a personality. When I was teaching English in Japan, I remember one of my female colleagues saying "Japan is full of bimbos". I know what she means. There's an image of ideal femininity over there that excludes personality, brains and assertiveness. You mustn't be strong. You should let your man protect you. You mustn't have a job that pays better than his. It's more or less people like Konoe, basically. If you married her, you'd go out of your mind with boredom.
I liked this show. It would have been a keeper had its OVA been a proper episode and if Shougo had married Mizutani. (Endings matter.) I won't be keeping it, but it's still good for twelve episodes and I like how fairly it plays. A set of clues is so clear and consistent that you'll be wondering if it's a double-bluff. At least for eleven episodes, it's one of the best harem anime I've seen.